4K-GM Cryocooler Performance and Thermal Conductivity of Ho x Er 1-x N
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4K-GM Cryocooler Performance and Thermal Conductivity of HoxEr1-xN
Takanori Nakano1, Yusuke Hirayama1, Takushi Izawa1, Takashi Nakagawa1, Yasushi Fujimoto2, Shinji Masuyama3, Toshio Irie4, Eiji Nakamura4, Takao A. Yamamoto1 1
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871,
Japan. 2 Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan 3
Oshima National College of Maritime Technology, 1091-1 Komatsu, Suo-oshima-cho,
Oshima-gun, Yamaguchi 742-2193, Japan. 4 Santoku Corporation, 4-14-34 Fukaekita-cho, Higashinada-ku, Kobe, Hyogo 658-0013, Japan.
ABSTRACT HoxEr1-xN (x=0.25, 0.5, 0.75) samples were synthesized by nitriding of HoxEr1-x alloy bars and their thermal conductivity κ were measured. The measured κ values were comparable to those of stainless steel and Er3Ni. Ho0.5Er0.5N showed the highest κ of the present three samples. The thermal diffusivity calculated from the κ and the specific heat indicates that Ho0.5Er0.5N is a very promising regenerator material for the cryocoolers.The electrical resistivity ρ was also measured as a function of temperature.
INTRODUCTION Gifford-McMahon (GM) cryocooler is a typical cryogenic refrigerator [1] widely used for helium and hydrogen liquefaction, and for systems using superconducting magnets such as diagnosis devices based on the magnetic resonance imaging. It generates cryogenic temperatures by the regenerator equipped in the system, and its cooling performance strongly depends on properties of the material packed in the regenerator. Required physical properties are high specific heat, C , and high thermal conductivity, N below 15 K, because the cool generated by the expansion of helium gas is desired to be stored and released as much and quickly as possible. In addition, the material should be shaped into spheres with smooth surface, because helium gas needs to flow with the least hydrodynamic resistance through the regenerator column packed with
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the spheres. Therefore, processing technology for fabricating the sphere is also of great importance. We have pointed out that rare earth nitrides [2-6] have high specific heats due to their magnetic transitions at cryogenic temperatures and they are regarded as the advanced regenerator material [7,8]. We executed cooling tests with a two-stage GM cryocooler packed with HoN and ErN spheres, and observed that both the materials made temperature below 4.2 K [9,10]. The cooling power, CP, obtained by ErN was found to exceed 0.1 W at 4.2 K, by using a commercial GM cryocooler of 0.1-W nominal CP [11]. Actually measured CP with the ErN sample was slightly smaller than with HoCu2. The Er sample included imperfections such as surface irregularity, pores in the bulk, and a considerable oxygen impurity, nevertheless the ErN sample was in keen competition with HoCu2. Its material processing is, of course, to be improved. We propose another improvement on its physical property by “alloying” of HoN and ErN. The specific heats of HoN and ErN are smalle
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